Tuning of Folding\nHeight in Bilayer-Folded Lamellae\nby Bending Rigidity Control

We investigated the bilayer-folded lamellar (<i>L</i><sub>f</sub>) mesophase appearing in the aqueous solution\nof amphiphilic\nrandom copolymers. A series of copolymers were synthesized by reversible\naddition–fragmentation chain transfer copolymerization of oligo(ethylene\nglycol) acrylate with alkyl acrylate with different alkyl chain lengths\nfrom octyl (C8) to octadecyl (C18). The alkyl acrylate composition\nwas adjusted between 50–60 mol %. In the concentrated solution\nwith the carbon number of the alkyl side chain higher than 10, the\ncopolymers associated in water via hydrophobic interaction between\nthe alkyl chains to produce micellar bilayers, which were periodically\nfolded into bilayer-folded lamellae. The appearance of a small-angle\nX-ray scattering (SAXS) peak at a low scattering vector corresponding\nto >10 nm length scale clearly distinguished the bilayer-folded\nlamellae\nfrom the micellar lamellae with the domain spacing of 5–7 nm.\nTwo-dimensional (2D) SAXS corroborated the presence of bilayer-folded\nlamellae developing perpendicularly to the micellar lamellae, which\nis consistent with our previous report. While the <i>L</i><sub>f</sub> phase was observed at room temperature for dodecyl (C12)\nand tetradecyl (C14) side chains that formed amorphous packing, crystalline\nhexadecyl (C16) and octadecyl (C18) chains seem to disturb bilayer\nfolding. Heating the solution above the melting temperature of the\nalkyl chains produced the <i>L</i><sub>f</sub> phase with\nthe largest folding height in the case of C16. The scaling relationship\nof the folding height to the carbon number supports the idea that\nthe bending rigidity of the bilayer influences the length scale of\nfolding.